AF 447 Search to resume

[Chris Scott]

Hi PJ2,

Re your item 4:
Wish we understood this "IVSI" better. Is it inertial, or barometric, or a fiendish combination?
Not really relevant, but – in response to your comment – one or two pilots on my (A320) fleet used to use the FPA (AP engaged) for routine descents. They achieved continuous idle thrust by asking the A/THR to try and achieve an IAS of, say, 250, when the selected FPA was good for, say, 300 at idle thrust.

Re items 6 and 7, the following is my best shot.

In still air, including the absence of thermals or downdrafts, the pitch angle minus the FPA represents the AoA (provided, I suggest, due allowance is made for what used to be called the "riggers angle of incidence" of the wing mounting; which is a constant).

In air which is moving vertically, there would be a correction to be made. For example, imagine a jet in the cruise, with pitch +3 and FPA 0. Alpha is +3. Now it enters an area of continuous updraft, and pitch has to be reduced to +2 to maintain FPA 0. But Alpha must have been maintained at +3. If Alpha had reduced to +2, the aeroplane would accelerate downwards due to loss of lift.

The case where the air mass is only moving horizontally ("wind"), also leads to an error. This may be best illustrated by looking at an extreme scenario. Imagine a microlight equipped with similar flight instrumentation to an Airbus (!). It is flying straight and level at a TAS of 50 kts into a 50-knot headwind (GS 0), pitch +3, FPA 0, Alpha +3. Because the aeroplane is stationary in relation to the earth, an FPA of zero would only be possible if the pilot maintains the VS precisely at zero. The tiniest rate of descent would presumably result in FPA -90 (vertical descent), suggesting an Alpha of +93. Similarly, a tiny rate of climb would give FPA +90 (vertical climb), suggesting Alpha -87. But, in fact, Alpha remains at +3.

Not having tried to work out any figures, I don't know how significant these two types of errors would be to a jet pilot, experiencing spurious airspeed indications at high altitude and high TAS, trying to use AoA to recover from an upset. Perhaps not very significant?

In the AF447 case, I presume the predominant upper wind in the area of the accident would have been fairly light; but the thermals and downbursts potentially severe.